The binding of Ca2+ to Ca2+-transporting microsomes derived from bovine uterine sarcoplasmic reticulum

An obligatory early step in the transport of calcium across the internal membranes of smooth muscle cells is the binding of calcium to the Ca,Mg-ATPase. The characterization of calcium binding to sarcoplasmic reticulum from smooth muscle has not been reported. Calcium binding to a bovine myometrium preparation was investigated using Scatchard analysis and a computer program utilizing weighted least squares curve fitting and an exact mathematical model of binding. This permitted objective measurement of goodness of fit and showed that best fit was obtained using a two site model. Magnesium did not change the affinity for calcium of the two sites; but reduced the number of low affinity sites to half.